Insulation depends on structure

Many insulating materials contain numerous small air spaces. Air conducts heat less readily than many solids, so these spaces can help reduce heat transfer. However, simply surrounding an object with a large volume of air does not necessarily provide equally effective insulation.

Air that is free to circulate can carry energy from one place to another. Dividing the air into small pockets limits this movement. The solid framework holds the pockets in place, although the framework itself also provides paths through which heat can travel.

Design therefore involves more than maximizing the amount of air. The material must retain its structure under expected conditions. If it is compressed, its pockets may shrink; if it becomes wet, water can replace some of the air. Either change can affect its performance.

This explains why a measurement made on a dry, undamaged sample is not a complete description of what will happen in use. A useful assessment also considers pressure, moisture, and maintenance. The same basic mechanism may operate in different products, while differences in structure determine how reliably each product preserves that mechanism over time.